Microwave tube



A... A L f Sept. 8, 1959 R. H. BARTRAM 2,903,619

MICROWAVE TUBE Filed Sept. 6, 1957 F/G. 3a

f la 36 F/G. 3b

INVEN TOR. HAL PH H. BARTRAM ATTORNEY 2,903,619 MICROWAVE TUin `My invention is directed toward strophotron oscillators. Y

A strophotron 'oscillator `is a multitransit electron tube adaptable for use in the VHF and UHF frequency ranges and `is described, for example, `in anarticle entitled New Electron Tube The Strophotron by Hannes `Alfven and Dag Romell, publishedin the Proceedings of the I.R.E:, v'ol. 42, No, 8, page 1239 August `1 954.

In myfcopendin'g application SerialNo. 680,'761, filed August 28, 1957; I disclosed anew fand improved stroph- 'ot-ron` provided with elongated accelerator having a uniform cross section defining an upper branch of an hyperbola. This strophotron rfurther includes first and second, separated plane Vreflectors so positioned as to coincide with corresponding portions of the first and second `asymptotes of the upper hyperbola branch, the .rliectors and the accelerator extending in the same direction. v Ille accelerator is maintained at a high positive potential relative 'to the reflectors, thus establishing an 'electric fie'ldtherebetween-.- This field, due to the geome- 'of theaccelerator and reflectors, creates yan hyper-v boli'e electric potential distribution within the entire re- `gion ,bounded by the accelerator and the reflectors.: Stat- `ed"difi`ere"ntly, the potential V vof any point Y'within this :region isgproportional to the quantity (y2-x2), where- Vin y is the vertical separation between this point "and the intersect-ion cfg-the first and second asymptotes and lector maintained at a highpositive potential relative to -the reflectors is positioned adjacent -theother end of the accelerator. A load, which lcan be for example a resistoror a resonant circuit tuned to the oscillator frequency, is coupled between the two reflectors.

fUnder these conditions, electrons emitted from-the cathodewill migrate'toward -the collector Ialong a curved rpath and exhibit the characteristic strophotron bethe projection of this path onto the plane of syinetr'yresembles a lti'och'oitL while the projection `f vonto `a second "plane perpendicular to `the symmetry and extemdingV in the direction of the ieflect`1s, resembles a v`damped sinusoifd having'an axis defined by the intersection of the two planes.

il-lowev, in common 'with the known strophotrong v1"thef'aibtfe cribed 'strophotron s'uiiers from certain tornrnon disadvantages. -For example, yfor 4a Vfixed cathdeiposiltiom (the `magnetic field intensi-ty required increases as the oscillation frequency increases,and evenittially 4the rrequired intensity b ecornes prohibitivelyjhig'h.

The required magnetic field intensity can be reduced Lfiice iby increasing the vertical separation between the cathode and the 'central point of the hyperbolically shaped portion of the accelerator. However, as separation is increased, the direct potential at the .point at which the emitted electrons intersect the plane of symmetry willdeerease and eventually will -be substantially equal to the direct reflector potential. The power output of the strophotron for a fixed tube geometry and values of `applied potentials, varies directly with the difference in direct potentialnbetween the reflectors and this point of intersection. Hence, as the vertical separation between the cathode and the central point of he hyperbolaportion decreases, the power output of stiophotron 'also decreases.

I Ahave invented a new type of stroph'otron'which overcomes these di'iculties Accordingly, it is an object of the present invention to rovide a new and improved strophotron of the character described,

Another object is to improve strophotron `operatim try-reducing the magnetic field Aintensity required for operation at any -given frequency Withinthe strophotron frequency range.

Still anotherobject yis to increase they-power output of a strophotron having a fixed tube geometry and fixed values vof applied potentials.

These and otherobjects of my invention veither be explained or will become apparent hereinatter.

In accordance with the principles of my invention, `I provide first and second separata-electrically conductive, elongated reflectors, each of which has a -unifornjl cross 'section defninga branch of an hyperbola. The refiectors `are placed parallel to each other and are so oriented as to respectively left hand 4and right hand 'branches of an hyper-bola. Hence, the two reflectors are disposed about a plane of symmetry which extends in rthe same direction as the accelerators and which passes lthrough `*the centralpoints of both hyperbola branches.

I further provide a first elongated, electrically conductivef-accelerator extending in the same directionjas the reflectors, the acceleratorjhaving a uniform cross ,section defining a branch of an-hyperbola. Thejaccelera- 1tor-is symmetrically disposed between the two reectors --in such manner that the first reflector and the accel- -Jner that the legs of the jme'mber respectively coincide with extensions of theffirst and second asymptotes, the vertex edge `of themember being substantially coincident with the intersection ofthe first and second-asymptotes. `The reflectors 'are maintained at a rst direct potential; the V-shaped 'member is maintained VVat a second -direct potential more positive than the first potential; the -accelerator4 is maintained at a third direct potential more positive than the second potential. The resultant 'electric field due to the 'geometry of the member, accelerator f'andreflectors, establishes an hyperbolic' electric potential distribution Within the electron interaction region bound- 'ed by -th'e r'nernber, accelerator and reflectors.- More 'particularly, the potential V at any point said region ispro'pl'tiohal tothe quantity @Q -x2) where y 4is the 'perpendicular separaticnbetween this point and th'everin a direction parallel to the plane of symmetry and perpendicular to the direction in which the reflectors extend.

A cathode is mounted in one of the reflectors at a point intermediate the central point of the said one reflector and the accelerator and adjacent one end of the accelerator. An anode or collector maintained at a high positive potential relative to the reflectors and the member is positioned adjacent the other end of the accelerator. A load is coupled between the two reflectors.

Under these conditions, electrons emitted from the cathode migrate toward the collector along a curved path confined between said one accelerator and the second plane of symmetry and will again exhibit the characteristic strophotron behavior; i.e. the projection of this path onto the plane of symmetry resembles a trochoid, while the projection of this path onto a plane perpendicular to the plane of symmetry and extending in the same direction as the electrodes resembles a damped sinusoid having an axis defined by the intersection of the two planes.

By virtue of this arrangement, for a given power output, the required magnetic field intensity can be substantially reduced over that hitherto required. Conversely, for a given field intensity, the power output can be substantially increased over that hitherto obtainable.

An illustrative embodiment of my invention will now be described with reference to the accompanying drawings wherein Fig. 1 is an isometric view of a strophotron in accordance with my invention;

Fig. 2 is a graph of the electric potential distribution between the accelerator, the reflectors and the V-shaped member shown in Fig. l; and

Figs. 3a and 3b are graphs of the motion of favorably phased electrons in the interaction region between the accelerator, the reflector and the V-shaped member.

` Referring now to Fig. 1, enclosed in an evacuated tube envelope (not shown) is an elongated, electrically conductive, accelerator 10. This accelerator has a uniform cross section dening an upper branch of an hyperbola having a central point 12.

Further provided are first and second, horizontally separated, electrically conductive, elongated reflectors 18 and 20 extending in the same direction as accelerator 10. Each reflector has a uniform cross section defining a branch of an hyperbola. The two reflectors are disposed about accelerator so as to define left hand and right hand hyperbola branches; the left hand branch having a central point 22, the right hand branch having a central point 24. Hence, the two reflectors are disposed about a plane of symmetry extending in the same direction as accelerator 10 and passing through central point 22 and Vertex 16.

Accelerator 10 together with each adjacent refiector 18 and define corresponding first and second common hyperbola asymptotes 26 and 28 therebetween. A V- shaped, electrically conductive member 14, vertically separated from accelerator 10 is interposed between reflectors 18 and 20 in such manner that legs 15 and 17 of member 14 coincide with extensions of asymptotes 26 and 28. The vertex edge 16 of member 14 is substantially coincident with the intersection of asymptotes 26 and 28.

A uniform magnetic field is established within the region ywhich includes the accelerator, the member, and the reflectors, the magnetic field vector pointing in a direction perpendicular to the direction in which accelerator 10 extends and to the plane of symmetry. (Means for establishing this field are conventional and are not shown here.)

Accelerator 10 is coupled to a point of positive direct potential |V1. The two reflectors 18 and 20 are coupled to a point of negative potential -V2. Member 14 is coupled to a point of ground or zero potential. As a result, the electric field established between the accelera-l tor, Athe member, and the reflectors esqblishes an hyperbolic electric potential distribution Within the electron interaction region. (Note that the accelerator, the reiiectors, and the V-shaped member are extended suiciently to substantially eliminate fringe electric fields.)

A cathode 34 is mounted in reflector 18 at a point intermediate the central point 22 of this refiector and accelerator 10, the cathode being adjacent one end of accelerator 10. An anode or collector 36 maintained for convenience at the same potential as accelerator 10, is positioned adjacent the other end of accelerator 10. A load 38 which can be purely resistive, but in this example is a resonant circuit tuned to the oscillation frequency, is coupled between reflectors 18 and 20.

Electrons are emitted from the cathode at an extremely low velocity and enter the interaction region. Many of these electrons then drift toward the collector along a curved path and exhibit the characteristic strophotron behavior.

More particularly, the projection of this path onto the plane of symmetry resembles a trochoid as shown in Fig. 3a. Further, the projection of this path onto a plane perpendicular to the plane of symmetry and extending in the same direction as the reflectors resembles a damped sinusoid as shown in Fig. 3b. The sinusoidal frequency is the oscillation frequency and is primarily determined by the electric field established between the accelerator electrode and the plane reflectors; the trochoid frequency is determined both by the magnetic and electric fields and is independent of the oscillation frequency. (The magnetic field intensity is adjusted to a value at which electrons are prevented from impinging on, and being collected by, the accelerator.)

The hyperbolic electric field distribution between the accelerator electrode, the refiectors and the member is plotted graphically in Fig. 2. As will be seen from Fig. 2, the electric potential V for any point P within the electron interaction region is proportional to the quantity (y2-x2), where y is the vertical separation between point P and a line 50 extending between central points 22 and 24 and x is the horizontal separation between point P and a line extending between central point 12 and the vertex edge 16 of member 14. The equipotential surfaces 54 are families of right hyperbolas with the lines y= lx as asymptotes. Note that since the accelerator defines a surface of -l-Vl potential and the reflectors define surfaces of -V2, the asymptotes 26 and 28 define lines of an intermediate potential V3. As indicated previously, member 14 is also at this intermediate potential V3.

The oscillation frequency, as stated previously, is determined by the electric field. Further, the electrons, in moving toward and away from the reflectors, induce an alternating voltage of oscillator frequency across the load, and this voltage acts on the electrons to produce the damping action. More specifically, when an electron leaves the cathode and enters the interaction region at such times as to be in phase with the induced voltage (such an electron is termed a favorably phased electron), it will oscillate back and forth between the plates in the manner indicated. The electrons which enter the interaction region at such times as to be out of phase with the induced voltage impinge on a reflector on the first or second pass and are thus removed from the interaction region. Hence, only the favorably phased electrons remain in the interaction region for an appreciable period, and the oscillator voltage appearing across the load is not substantially affected by out of phase electrons.

The amplitude of the sinusoidal motion of any electron is determined by the vertical displacement between this electron and the central point of the appropriate hyperbola branch, the amplitude decreasing as the displacement increases.

ljlowever, by virtue of the hyperbolic electric field distribution the oscillation frequency remains constant despite changes of vertical displacement.

The initial amplitude of the sinusoidal motion of any electron is determined by the vertical displacement between this electron and the central point of the appropiate hyperbola branch, the amplitude decreasing as the displacement increases.

However, by -virtue of the hyperbolic electric field distribution the oscillation frequency remains constant despite changes of vertical displacement.

It rwill be seen from a study of Figs, 1 and 2, that the potential at the point at which the electrons emitted from cathode 34 intersect the second plane will vary between the limits of -j-Vl and V3 depending upon the separation between cathode 34 and the central point 12 of accelerator 10.

In the prior art of which I am aware, the reflectors are maintained at potential V3; hence, in this situation as the cathode is moved away from the accelerator, the potential of the point of intersection of the second plane approaches V3 and the power output (which varies directly with the difference in potential between this point and the reflectors) 'approaches zero. In contradistinction, in my invention, the reflectors are maintained at a potential of -V2, while the V-shaped member is maintained at. potential V3; as the cathode is moved away from the accelerator, the difference in potential and hence the power output decreases at a much slower rate and ultimately attains a reasonably large finite value.

Further, for a given power output, the conventional cathode-accelerator separation must be substantially smaller than the separation required in my invention, and hence, the required magnetic field intensity in my invention under these conditions is substantially less than that required by the prior art.

While I have shown and pointed out my invention as applied above, it will be -apparent to those skilled in the art that many modifications can be made within the scope and sphere of my invention as defined in the claims which follow.

What is claimed is:

1. In a strophotron, a first electrically conductive elongated accelerator extending in a given direction and having a uniform cross section defining an upper branch of an hyperbola; first and second electrically conductive reflectors extending in said direction and separated from each other and said accelerator, said first and second reflectors having uniform cross sections respectively defining left hand and right hand hyperbola branches, the accelerator and each of said reflectors respectively defining first and second common asymptotes, said first reflector having a slot adjacent one end; a cathode mounted within said slot; a V-shaped electrically conductive member havingr its vortex coincident with the intersection of said asymptotes and having each of its legs coincident with a corresponding one of said asymptotes; a collector positioned adjacent said member, said reflectors and said accelerator at the end of said first reflector remote from said slot; means coupled to said accelerator to establish a first direct potential with respect to a reference potential thereon; means coupled to said member to establish said reference potential thereon; means coupled to said reflectors to establish a second direct potential with respect to said reference potential thereon, said first potential exceeding said reference potential, said second potential being less than said reference potential; and means to establish a time invariant magnetic field Within a region 6 surrounding said accelerator, said member and said re-` flector, the magnetic field vector pointing perpendicular to a plane of symmetry extending in said direction and passing through said vertex and the central point of the upper hyperbola branch.

2. ln a strophotron, a first electrically conductive elongated accelerator extending in a given direction and having a uniform cross section defining an upper branch of an hyperbola; first and second electrically conductive reflectors extending in said direction and separated from each other and said accelerator, said first and second reflectors having uniform cross sections respectively defining left hand and right hand hyperbola branches, the accelerator and each of said reectors respectively defining first and second common asymptotes, said first reflector having a slot adjacent one end; a cathode mounted within said slot; a V-shaped electrically conductive member having its vertex coincident with the intersection of said asymptotes and having each of its legs coincident with a corresponding one of said asymptotes; a collector positioned adjacent said member, said reflectors and said accelerator at the end of said first reflector remote from said slot; means coupled to said accelerator to establish a first direct potential with respect to a reference potential thereon; means coupled to said member to establish said reference potential thereon; means coupled to said reflectors to establish a second direct potential with respect to said reference potential thereon, said first potential exceeding said reference potential, said second potential being less than said reference potential.

3. In a strophotron, a first electrically conductive elongated accelerator extending in a given direction and having a uniform cross section defining an upper branch of an hyperbola; first and second electrically conductive reflectors extending in said direction and separated from each other and said accelerator, said rst and second reflectors having uniform cross sections respectively delining left hand and right hand hyperbola branches, the accelerator and each of said reflectors respectively defining first and second common asymptotes, said first reflector having a slot adjacent one end; a cathode mounted within said slot; a V-shaped electrically conductive member having its vertex coincident with the intersection of said asymptotes and having each of its legs coincident with a corresponding one of said asymptotes; a collector positioned adjacent said member, said reflectors and said accelerator at the end of said first reflector remote from said slot; and means to establish a time invariant magnetic field within a region surrounding said accelerator, said member and said reflector, the magnetic field vector pointing perpendicular to a plane of symmetry extending in said direction and passing through said vertex and the central point of the upper hyperbola branch.

References Cited in the le of this patent UNITED STATES PATENTS 2,124,270 Broadway July 19, 1938 2,293,567 Skellett Aug. 18, 1942 2,414,121 Pierce Jan. 14, 1947 2,520,813 Rudenberg Aug. 29, 1950 2,536,150 Backrnark et al. Jan. 2, 1951 2,834,908 Kompfner May 13, 1958 2,844,753 Quate July 22, 1958 FOREIGN PATENTS 729,930 Great Britain May 11, 1955 

